2007 VW Jetta starts at $16,490 with the 2.5 liter. 2.0T is a little out of your pricerange: $22,110 - but that's a really stellar vehicle.
2007 Subaru Legacy Special Edition starts at $20,795. AWD is probably not a big priority in CA, but this is a very nice vehicle. I can't stand FWD, so this one would be near the top of my list.
2007 Subaru Impreza 2.5i starts at $17,995. The WRX is slightly out of your stated price range, but even the base model is a solid vehicle - fun to drive.
2007 Chevrolet Cobalt SS starts at $18,850 - A very well-respected performance car in this price range. I'm not a domestic fan, but this is a nice car.
2007 Pontiac G6 starts at $17,825 - A bit larger than the others I have listed - a better choice if you need more space.
The Mazda would also be a good choice. At this stange in the process, I suggest you just go out a testdrive a bunch of cars, including the ones I have listed, to see what you like. Car buying is a very subjective activity, so you will only end up with the best vehicle for you if you take the time to sample all the possibilities firsthand. Good luck with your shopping process.
